1. Field of the Invention
This invention relates to the optical measurement methods in mechanics using scattered light techniques. Specifically, the invention relates to opto-mechanical apparatus which is useful for the rapid, accurate and theoretically correct measurements of the stress-induced birefringence, and, in particular, to determine cross-sections through elastic isodynes which carry information on the normal and shear stress components.
The measurements are simple when the patterns of light scattering are close to the Rayleigh model of scattering.
2. The Prior Art
There are several known methods of birefringence measurements using light scattered along the path of a narrow light beam (laser light), or a thin light sheet (laser light sheet). Depending on the technique of measurements one can distinguish the point-wise, line-wise, and whole field measurements (recordings). The point-wise measurements are usually expensive and require a costly measurement system. The line-wise measurements are usually performed in such a manner that the observation angle (see FIG. 2), vary from point to point along the path of the light beam; in addition, the location of the light beam in a body can not be recorded simultaneously on the same recording. The influence or existence of optical noise can not be detected and/or determined. The whole field measurements (recordings) are performed at the observation angles and azimuthal angles varying from point to point. Usually the image plane is not parallel to the selected object plane. As a result the intensity of scattered light is additionally modulated by the alterations of the observation angles and azimuthal angles; this unintentional modulation obscures the meaning of the recordings; in addition major portions of the intended object plane are out of focus. This leads to a recording of fringes, the intensity and geometry of which depends not only on the birefringence, but also on an undetermined transfer function of the measurement system and chosen experimental set-up. The collecting prism developed by J. T. Pindera improves incompletely the situation, since it inherently introduces an additional light modulation and a selective transmittance, which impair the signal/noise ratio.